U.S. patent application number 12/436625 was filed with the patent office on 2010-11-11 for model based method for selective catalyst reducer urea dosing strategy.
Invention is credited to Zhiping Han, Wolfgang Krueger, Bryant C. Pham, Amr M. Radwan, Sathish Sankara-Chinttoanony, Min Sun.
Application Number | 20100281855 12/436625 |
Document ID | / |
Family ID | 43061513 |
Filed Date | 2010-11-11 |
United States Patent
Application |
20100281855 |
Kind Code |
A1 |
Sun; Min ; et al. |
November 11, 2010 |
MODEL BASED METHOD FOR SELECTIVE CATALYST REDUCER UREA DOSING
STRATEGY
Abstract
A method to control NO.sub.x slippage in an electronic
controlled internal combustion engine exhaust system equipped with
a selective catalyst reducer (SCR) and a urea doser.
Inventors: |
Sun; Min; (Troy, MI)
; Sankara-Chinttoanony; Sathish; (Westland, MI) ;
Pham; Bryant C.; (Canton, MI) ; Radwan; Amr M.;
(Canton, MI) ; Han; Zhiping; (La Salle, CA)
; Krueger; Wolfgang; (Farmington, MI) |
Correspondence
Address: |
RADER, FISHMAN & GRAUER PLLC
39533 WOODWARD AVENUE, SUITE 140
BLOOMFIELD HILLS
MI
48304-0610
US
|
Family ID: |
43061513 |
Appl. No.: |
12/436625 |
Filed: |
May 6, 2009 |
Current U.S.
Class: |
60/286 ;
60/299 |
Current CPC
Class: |
F01N 2900/1616 20130101;
Y02T 10/12 20130101; F01N 2560/02 20130101; F01N 2570/14 20130101;
F01N 2610/02 20130101; Y02A 50/20 20180101; Y02T 10/24 20130101;
Y02A 50/2344 20180101; F01N 3/208 20130101; F01N 2900/1622
20130101; Y02A 50/2325 20180101 |
Class at
Publication: |
60/286 ;
60/299 |
International
Class: |
F01N 9/00 20060101
F01N009/00; F01N 3/10 20060101 F01N003/10 |
Claims
1. A method to control NO.sub.x slippage in an electronic
controlled internal combustion engine exhaust system equipped with
a selective catalyst reducer (SCR) and a urea doser, comprising:
determining the SCR operating condition; determining engine out
NO.sub.x exhaust flow rate into the SCR; adapting urea dosing
conditions to conform to the SCR operating condition; determining
ammonia storage, ammonia slip and NO.sub.x conversion in the
exhaust gas flow out of the SCR; and recalibrating the SCR
operating condition in response to pre-targeted ammonia storage,
ammonia slip, and NO.sub.x conversion.
2. The method of claim 1, wherein said SCR operating condition is
determined by using temperature of said exhaust and the exhaust
flow rate through the SCR to determine SCR age.
3. The method of claim 2, wherein the SCR reduced age is determined
by the amount of time the SCR operates above a predetermined
temperature.
4. The method of claim 3, wherein the predetermined temperature is
in the range of from about 500.degree. C. to about 700.degree.
C.
5. The method of claim 2, wherein temperature operation of the SCR
is contained as data points within a map or table of an electronic
control module memory.
6. The method of claim 5, wherein the SCR condition is predictable
by the electronic controller based upon data contained in the map
or table.
7. The method of claim 1, wherein said urea dosing is controlled by
determining the amount of ammonia slippage in the SCR exhaust gas
flow.
8. The method of claim 1, wherein said urea dosing is controlled by
an engine control module having memory and urea control strategies
resident therein.
9. The method of claim 1, wherein the engine is a diesel engine and
adapting urea dosing conditions to current SCR condition includes
considering at least engine air mass flow rate, engine total air
flow rate; engine NO.sub.x flow rate; SCR inlet N0.sub.2 over
NO.sub.x ratio; SCR inlet exhaust pressure; SCR inlet temperature;
diesel oxidation temperature; ambient air temperature; diesel
particulate filter oxygen flow rate and vehicle speed to develop
ammonia rate for urea dosing control.
10. The method of claim 1, wherein ammonia dosing rate is
controlled by targeting a critical ammonia slip.
11. The method of claim 1, wherein ammonia dosing rate is also
controlled by targeting a critical ammonia storage in the SCR to
optimal SCR de-NO.sub.x efficiency and prevent ammonia slip during
step acceleration of the vehicle and various operating
conditions.
12. The method of claim 1, wherein the engine is operated for a
predetermined period of time to determine a stable engine operating
condition and ammonia slip.
13. The method of claim 1, wherein ammonia storage, ammonia slip,
NO.sub.x reduction efficiency may be modeled under one dimension
SCR model and determining desired urea dosing rate with a desired
ammonia storage and ammonia slip is modeled with a one dimension
SCR inverse logic model.
Description
TECHNICAL FIELD
[0001] Emission control for compression or diesel engines has been
a subject of great interest, especially with the advent of new
emission control regulations and the need to operate cleaner
engines to reduce overall global pollution levels. As a part of
this effort, many diesel engine manufacturers have resorted to
using exhaust system after treatments that include Diesel
particulate filers to trap particulate emissions and hydrocarbons,
a diesel oxidation catalyst to convert NO.sub.x to N.sub.2, (HC to
H.sub.20 and Co.sub.2) and a selective catalyst reducer with a urea
doser to trap NO.sub.x in the SCR until operating conditions of the
SCR permit the NO.sub.x to be treated with exposure to ammonia,
such as from urea, to change NO.sub.x to N.sub.2 gas for emission
to the atmosphere.
[0002] There is a need for a model based method for developing and
implementing a SCR urea dosing strategy.
BACKGROUND
[0003] It has become understood that SCR performance will degrade
over time, so that as an SCR ages, it is less efficient than it was
when installed new in the vehicle exhaust system. In order to
maintain required emission standards, it has become necessary to
understand the aging process of the SCR and how to adapt the engine
operation, particularly the urea dosing to a strategy that takes
into account the age of the SCR.
[0004] When the SCR is operating at low temperature, ammonia is
absorbed by the SCR, whereas at high temperatures, there is an
increased ammonia slip past the SCR. At low temperatures, it is
desirable to have a very high storage of ammonia in the SCR. At
high temperature, it is desirable to have low ammonia storage in
the SCR. It has been determined that SCR ages as a function of
temperature of operation. It has been determined that the storage
capacity of the SCR for ammonia degrades with SCR age. As the
temperature of the SCR rises to about 500.degree. C. or more, the
performance degrades. Understanding the amount of time the SCR
operates above a predetermined temperature can be used to map or
populate a data table with expected levels of SCR efficiency, so
that NO.sub.x and ammonia are not vented to the atmosphere, and so
that a warning alert may be made to the vehicle operator once it is
determined that the SCR is too old to be effective. Such
information may be developed using a map or data points in a table.
The map or data points may further be developed according to a one
dimension model of the operation of the SCR and a one dimension
model inverse logic model for the SCR. There is a need for a method
to determine how urea dosing can be adjusted and the engine exhaust
gas flow will meet emission standards regardless of the age of the
SCR.
SUMMARY
[0005] In one embodiment, the present application is directed to a
method to control NO.sub.x slippage in an electronic controlled
internal combustion engine exhaust system equipped with a selective
catalyst reducer (SCR) and a urea doser. One method includes
determining the SCR operating condition; determining engine out
NO.sub.x exhaust flow rate into the SCR; adapting urea dosing
conditions to conform to the SCR operating condition; determining
ammonia storage, ammonia slip and NO.sub.x conversion in the
exhaust gas flow out of the SCR; and recalibrating the SCR
operating condition in response to ammonia storage slip and
NO.sub.x conversion.
[0006] In another embodiment, the method may include determining
the SCR operating condition by using temperature of the exhaust and
the exhaust flow rate through the SCR to determine SCR age.
Generally, the SCR reduced age may be determined by the amount of
time the SCR operates above a predetermined temperature. More
particularly, the predetermined temperature is in the range of from
about 500.degree. C. to about 700.degree. C.
[0007] The temperature operation of the SCR may be contained as
data points within a map or table of an electronic control module
memory, and the SCR condition is predictable by the electronic
controller based upon data contained in the map or table.
[0008] When the SCR has reached a stable operating condition, urea
dosing may be controlled by determining the amount of ammonia
storage and slippage in the SCR exhaust gas flow. Generally, the
urea dosing may be controlled by an engine control module having
memory and urea control strategies resident therein.
[0009] When the engine is a compression ignition or diesel engine,
adapting urea dosing conditions to current SCR condition includes
considering at least engine air mass flow rate, engine total air
flow rate; engine NO.sub.x flow rate; SCR inlet NO.sub.2 over
NO.sub.x ratio; SCR inlet exhaust pressure; SCR inlet temperature;
diesel oxidation temperature; ambient air temperature; diesel
particulate filter oxygen flow rate and vehicle speed to develop
ammonia rate for urea dosing control.
[0010] The ammonia dosing rate of the SCR is controlled by
targeting both critical ammonia slip and ammonia storage in the SCR
and is targeted to prevent ammonia slip during step acceleration of
the vehicle and may vary based on operating conditions. Generally,
the engine is operated for a predetermined period of time to
determine a stable engine operating condition and ammonia slip.
Ammonia storage, ammonia slip, NO.sub.x reduction efficiency may be
modeled under one dimension SCR model and desired urea dosing rate
with a desired ammonia storage and ammonia slip may be modeled with
a one dimension SCR inverse logic model.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a schematic representation of an engine with an
exhaust system including a diesel particulate filter (DPF) a
selective catalyst reducer (SCR) and a diesel oxidation catalyst
(DOC).
[0012] FIG. 2 is a representation of a model based open loop SCR
control system I/O.
[0013] FIG. 3 is a representation of a model showing how ammonia
dosing rate is determined.
[0014] FIG. 4A is a graph showing ammonia storage in the SCR as a
function of SCR temperature
[0015] FIG. 4B is a graph showing ammonia storage in the SCR as a
function of time and temperature of the SCR.
[0016] FIG. 5 is a graph demonstrating a model based SCR Control at
step acceleration condition.
[0017] FIG. 6 is graph demonstrating a One Dimension ammonia
storage distribution based upon SCR inlet temperature and time.
[0018] FIG. 7 is a graph showing model based SCR Control at
transient and steady state conditions.
[0019] FIGS. 8A and 8B is a graph showing Constant Dosing Alpha
Strategy ammonia slip.
[0020] FIGS. 8C and 8D form a graph showing model based dosing
strategy ammonia slip according to one embodiment of the present
disclosure.
[0021] FIG. 9A is a graph showing SCR age as a function of SCR
Temperature
[0022] FIG. 9B is a graph showing SCR deNO.sub.x efficiency as a
function of SCR aging function time.
[0023] FIG. 10 is a software flow diagram showing one method
according to the present disclosure.
DETAILED DESCRIPTION
[0024] Turning now to the drawings wherein like numbers refer to
like structures, FIG. 1 schematically illustrates a compression
ignition engine 10 for an on-highway vehicle 12. The engine 10
includes an engine control module 14 that controls operation of the
engine 10 and also controls exhaust component urea dosing according
to the present invention as described below.
[0025] Exhaust manifold sensors 16 and tail pipe sensors 18 provide
information to the engine control unit (ECU) 14, that may be
comprised of an engine control module and a component control
module in communication with each other over an engine common area
network (ECAN) that is used to ensure that the component control
module and the ECU functions in a coordinated manner to operate the
engine and attendant systems. The ECU controls the engine and the
exhaust component operation, including urea dosage as will
hereinafter be described.
[0026] The exhaust manifold sensors 16 may provide information
regarding NO.sub.x levels, air/fuel ratios, temperature, and
pressure at any of the exhaust system components. More
specifically, the exhaust manifold sensors 16 and tail pipe sensors
18 may provide information regarding NO.sub.x, and temperature that
enable the ECU to detect an impending need for ammonia storage in
the SCR or urea dosage. The ECU may also monitor other engine
operating parameters to determine the need for urea dosage or
ammonia storage. For example, the ECU may contain data tables or
maps populated with data. The map or data points may further be
developed according to a one dimension model of the operation of
the SCR and a one dimension model inverse logic model for the SCR.
The ECU, based upon input from sensors at the SCR inlet and SCR
outlet uses the tables or maps to determine how urea dosing can be
adjusted and the engine exhaust gas flow will meet emission
standards regardless of the age of the SCR. The exhaust system is
seen with conduit 19 and particulate filter 22, catalyzed soot
filter 24, or NO.sub.x absorber catalyst, such as the SCR 20. Urea
doser 26 is in close proximity to the SCR inlet for the
administration of urea according to a method of the present
disclosure. A warning light 28 may be provided to alert an operator
that the SCR is too old to operate efficiently and should be
replaced.
[0027] Turning to FIG. 2, there is illustrated a model based open
loop SCR control System I/O 30 according to one embodiment of the
present disclosure. Specifically, the model illustrates that engine
air mass flow rate 32, engine total air flow rate 34, engine
NO.sub.x flow rate 36, SCR inlet NO.sub.2 over NO.sub.x ratio 38,
SCR inlet pressure 40, SCR inlet temperature 42, DOC inlet
temperature 44, ambient temperature 46, 02 flow rate from diesel
particulate filter (DPF) 48, and vehicle speed 50 are input into
the model. The model considers sensor input indicative of ammonia
storage of the SCR 52, ammonia slip from the SCR 54, SCR outlet
NO.sub.x 56, SCR deNO.sub.x efficiency 58 and the requested ammonia
rate in order to determine and the ammonia rate for dosing and
thereby control the urea doser to ensure that the proper amount of
urea is used at all stages of the SCR operation as indicated at
59.
[0028] FIG. 3 is a schematic representation of model 60 showing the
inputs as described in relation to FIG. 2 above, and their
consideration by a one dimensional model 62 that then inputs its
determinations to model inversion 64 which, together with the input
regarding critical ammonia storage and slip 66, is considered in
the model inversion 64 to determine ammonia dosing rate 59. Note
that the ammonia dosing rate is in a feedback loop with the one
dimensional SCR model 62 as an input therein. Generally, the urea
dosing rate is controlled by targeting the critical ammonia storage
and slip in the model schematically presented in FIG. 3.
[0029] Specifically, one example to explain the inverse logic of a
one dimensional SCR model may be represented by the equation
(1)
aX.sup.2+bX+(c-Y)=0
[0030] Wherein
a=f.sub.a(T,time.sub.resi)
b=f.sub.b(.theta..sub.stor,.sup.c.sub.nax)
c=f.sub.c(ratio.sub.NO2,C.sub.02)
.theta..sub.stor=f.sub..theta.(t,T,time.sub.resi,ratio.sub.NO2,C.sub.02,-
C.sub.NOx,C.sub.NH3 . . . )
[0031] One example of the inverse model, as depicted in FIG. 3, may
be represented by the equation
X = - b .+-. b 2 - 4 a ( c - Y ) 2 .differential. ##EQU00001##
[0032] wherein the variables have the same values as set forth in
regard to equation (1) above.
[0033] .theta.=1 is the ammonia storage capacity of the SCR. If the
SCR is fully stored with ammonia, there will be ammonia slippage
from the SCR. The higher the ammonia storage levels, the higher the
conversion of ammonia and NO.sub.X to N.sub.2 will occur, but there
will also be higher ammonia slip past the SCR. In operation, based
upon engine and SCR conditions, a particular ammonia storage level
is targeted so that there can be a higher NO.sub.X conversion rate
to N.sub.2, thereby reducing ammonia slippage.
[0034] FIG. 4A is a graph showing ammonia storage capacity in the
SCR as a function of SCR temperature, based upon the model
developed according to one embodiment of the present disclosure.
Specifically, model data points 70, 72, 24, 76 and 78 form a curve
80, that is almost identical with observed data points 82, 84, 86,
88 and 90 which form an almost identical curve 92 as curve 80. This
correlation indicates that the model is a very good predictor of
ammonia storage as a function of SCR temperature, and may be relied
upon instead of the actual observed data points.
[0035] FIG. 4B is a graph showing ammonia storage level in the SCR
as a function of time and temperature of the SCR. It can be seen
that as SCR inlet temperature 92 increases to a spike 93 of about
400.degree. C., ammonia storage 94 increases until the SCR inlet
temperature reaches about 400.degree. C., at which point 95 ammonia
storage decreases, and-ammonia slippage increases. Considering the
data from the two graphs of FIGS. 4A and 4B, it may be seen that
ammonia storage should be limited to prevent ammonia slip past the
SCR during step-acceleration operation of the vehicle. The graph
shows that the NH.sub.3 dosing strategy is best determined by
noting when the NH.sub.3 slip is equal to NH.sub.3 slip_critical
93, should be that ammonia slippage should equal ammonia
slip_critical and the NH.sub.3 storage 96 is less than or equal to
ammonia storage critical
[0036] FIG. 5 is a reading of a model based SCR control at step
acceleration condition. Basically, the graphs show SCR substrate
temperature, dosing alpha, deNO.sub.x efficiency, ammonia slippage
past the SCR and ammonia storage percent. It can be seen that under
dosing due the lower deNO.sub.x efficiency results in higher
ammonia storage critical, whereas overdosing due to ammonia
oxidation results in an increase in the ammonia slip critical.
[0037] FIG. 6 is graph demonstrating a One Dimension ammonia
storage distribution based upon SCR inlet temperature and time. It
can be seen that as the SCR inlet temperature changes from 200 to
350.degree. C., at 2000 RPMS, ammonia storage distribution
decreases and assumes an almost steady state as indicated at
97.
[0038] FIG. 7 is a graph showing model based SCR Control at
transient and steady state conditions. Note that when the SCR
substrate reaches a predetermined temperature, in this case of
about 350.degree. C., the dosing alpha, deNO.sub.x efficiency
ammonia slip and ammonia storage percentage each assumed a steady
state, as indicated at 81, 83, 85 and 87 respectively.
[0039] FIGS. 8A and 8B are graphs showing Constant Dosing Alpha
Strategy ammonia slip. As seen therein the dosing alpha is equal to
1, and ammonia slip past the SCR depends upon cycles. As is
apparent in the graphs, a longer low temperature period permits
higher ammonia slip past the SCR. The graph 100 is comprised of two
parts. Section 102 is the temperature of the SCR over operating on
engine and 104 is the temperature of the SCR in Celsius. Section
106 is NH.sub.3 slip as measured in parts per million 108. Time in
seconds is shown at 110. As can be seen by reference to graphs 8A
& 8B, as CR temperature increases to beyond about 650.degree.
C., the NH.sub.3 slip, as measured in ppm past the SCR spikes, and
then decreases, and then decreases as the SCR temperature decreases
due to dosing with fuel. In addition, the longer the period of time
the SCR remains at a low temperature, the greater the ammonia slip
past the SCR. In addition, ammonia slip past the SCR is independent
of engine operation. Rather, it is dependent upon temperature of
the SCR.
[0040] FIGS. 8C and 8D form a graph showing a model based dosing
strategy ammonia slip according to one embodiment of the present
application. Specifically, the model shows that as SCR temperature
passes approximately 650.degree. C., the NH.sub.3 slippage spikes,
and decreases when the SCR temperature is reduced. Moreover, the
model further shows that the NH.sub.3 slip is independent of engine
cycle time.
[0041] FIG. 9A is graph 112 showing a model of SCR aging as a
function of SCR Temperature. The X axis 114 is SCR temperature in
Celsius, and the Y asix 116 is the SCR aging as a function of SCR
temperatures. Basically, the aging of the SCR may be presented by
the equation:
Age.sub.SCR=.SIGMA.factor.sub.aging.sub.--.sub.equiv.times.t.sub.step
[0042] Using the formula, it is possible to create a SCR aging
factor function based on SCR aging test results by assuming aging
factor is unit at 700.degree. C., and normalize aging rate at other
temperatures to establish a correlation between SCR age and
NO.sub.x reduction efficiency. [0043] The plots above are not from
test data, for explaining the concept only
[0044] FIG. 9B is a graph 118 showing SCR deNO.sub.x efficiency as
a function of SCR aging time. To create a SCR aging factor function
based on actual SCR test results, it is helpful to assume that the
aging factor is a predetermined temperature, in this case, the unit
is at about 700.degree. C. The SCR aging rate may be normalized at
other temperatures as well. A correlation between the SCR age and
the NOX reduction efficiency is established and the plot 120 set
forth in FIG. 9A indicates that as SCR Temperature rises, the SCR
aging factor rises as well. Similarly, FIG. 9B the plots 122, 124
and 126 indicate that when the SCR is operated at 700.degree. C.,
600.degree. C. and 500.degree. C. respectively, the deNO.sub.x
efficiency decreases as the SCR aging cycle time advances.
[0045] FIG. 10 is a software flow diagram showing one method 128
according to the present disclosure. Specifically, step 130 is
determining the condition of the SCR. In this regard, temperature
and time operated at specific temperature above a predetermined
temperature are factors that are considered. Step 132 is
determining engine out NOx flow rate into the SCR. This may be
accomplished by sensor input at the SCR inlet. Step 134 is adapting
a urea dosing condition to current SCR conditions, according to the
model and inverse models as set forth above. Step 136 is determine
the ammonia slip, and NO.sub.x conversion at the SCR and step 138
is recalibrate the SCR condition based upon operating conditions,
and the software loops back to step 130.
[0046] The words used in the specification are words of description
and not words of limitation. Many variations and modifications are
possible without departing from the scope and spirit of the
invention as set forth in the appended claims.
* * * * *